Hi T, I bought a pail of plaster a couple years ago, haven't opened it and hope not to. It was my standard method on previous layouts, but the few scenes I've done on the present layout have used stacked and contoured foam, and I really like it. I don't do anything unique or original, I use a surform tool to contour the landform. I don't leave any area smooth. Even the flat areas get a couple passes to roughen them up. I then paint with earth latex and sprinkle on dirt and ground foam, same as most everyone. I like the foam especially for the next steps, which are planting weeds and trees. It is easy to poke holes with an awl or something similar, plant the tree and often no need for glue, as the foam tends to expand a bit and hold the tree. With plaster I used to have to drill a hole and then vacuum up the white powder. And glue the tree in. And repair the area with ground cover. When I have areas which need to be filled in (cracks around bridge abutments, etc.) instead of plaster or spackle I use sawdust. I throw some in an old spackle container, pour in the earth colored latex paint and some carpenters glue, mix well and apply. It stays soft for about 24 hours, allowing planting of weeds, etc. Once it's hard, it really hard.
Gary